High [CVE-2026-9099] Group-Admin Escalation to Realm-Admin

A flaw was found in Keycloak. A missing authorization check in the GroupResource.addChild() endpoint within the Admin REST API allows an authenticated user with limited administrative privileges to reparent any existing group.

When Fine-Grained Admin Permissions v2 (FGAPv2) is enabled, an attacker with management rights over a single low-privilege group can reparent a highly privileged group (such as one possessing the realm-admin role) under their managed group.

Because group permissions follow a hierarchical structure, this action unauthorizedly grants the attacker management and password-reset capabilities over the members of the targeted privileged group.

An attacker can exploit this to reset an administrator's password, compromise the account, and achieve a full realm takeover, leading to a complete compromise of confidentiality, integrity, and availability. This vulnerability is rated as High impact.

When Fine-Grained Admin Permissions (FGAPv2) are enabled in Keycloak, a delegated administrator with specific `manage-members` permissions on a low-privilege group can bypass authorization checks to reparent any other group, including those with `realm-admin` roles. Red Hat severity: Important — CVSS 7.7 (CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:N).

Weakness: CWE-639.